Some people like the idea of a Trojan being implemented in the game.
One of the ways that it should work I'm guessing should be through email but it also mean the email construct might change a bit, such as the option to open the message or deleted but if there was a file attached and if there was a Trojan attached and the message was open the system will be infected without them knowing although they will get a message probably unrelated to whatever. The Trojan should work like this:

You can use HC and BB to make the Trojan, Trojan are for Pure destruction or delay work such as slower upgrading. Trojans can be handcrafted allowing the player to choose how it will affect the target an example of this would be choosing how slow the upgrades become, but that would involve the use of HC, But it another story with BB.
In the regard of BB it will cost 1000BB to just to start first level of Destruction. HC is just use to put Viruses in their gateway without hacking them and slowing upgrades, need at least 10,000,000 HC for one virus, yeah only the desperate will think of this idea. Let get to the 4 stages of Destruction

First level of Destruction: The Eye. Cost 1000BB or 1,000,000,000HC. it is permanent on Gateway until target change gateway or until the creater remove it as target but you will lose 100 rep if you remove it.

The First stage will be Spying on target gateway this is limited however because you can only see what is happening but you can steal IPs that show up in logs without the fear of being hacked or blacklisted. you can see
There messages, software, and hardware.

Second level of Destruction: HBStealing: Cost 2000BB or 2,000,000,000HC. It can removed by a very strong AV or by creater.

the second stage will simply be stealing from a gateway and yes that what I said stealing from a gateway, basically allowing the creator of the Trojan to take software or files(not packages or purchases) that the player work so hard to achieve such as: back-tracking, cracker, Firewall. Anything else will be an extra cost of 100 BB for each software stolen, in the process of stealing software the target software will reset to half of what it was.You can steal any file that is in Downloads only, but keep that a Secret.

Third level of Destruction: CPController: Cost 5000BB or 5,000,000,000HC. it is permanent on Gateway until target change gateway, Creater can remove. Can be useful with allies.

Allow control of target Gateway, it enables you to have complete control of the target gateway while they are unaware. You will be able to hack from their Gateway and if you want you can upload packages or apps that will work to their Gateway as a gift.(or viruses)

Forth level of Destruction: GWDeath: Cost 20,000BB or 20,000,000,000HC. it is permanent on Gateway until Gateway dies. can not be remove. DANGEROUS!!!

Will act as FBI program and try to Take target Gateway. Taking all of the Target HC(Not BB) and target Gateway will be REMOVED. (There is a consequence in using this app once you use it you will be put on missions list and will be hunted down as a criminal.) this app is used for Revenge if you just can't get enough.(or if you are just a bully)

Requirements in order to use these apps: must purchase Trojan.App in shop to have access to these Trojans.